The present invention relates to a process for phosphating metals, in particular steel and galvanized steel, by a spray method, using an aqueous acidic phosphating solution containing an accelerator, zinc and, if appropriate, nickel, and the use of this process prior to electrocoating.
German Offenlegungsschrift No. 2,232,067 describes aqueous acidic phosphating solutions, having a weight ratio of Zn:PO.sub.4 of 1:(12 to 110) for the surface treatment of metals, in particular iron and steel. The zinc content, which is lower than that of the conventional phosphating baths, leads to improved thin, uniform phosphate coatings which are very firmly bonded and resistant, and particularly suitable as a basis for subsequent electrocoating.
PCT publication No. WO 84/00386 discloses phosphating solutions which contain 0.2 to 0.6 g/l of Zn and Ni in a ratio of 5.2 to 16 moles of Ni per mole of Zn, and which give phosphate layers which are particularly resistant to dissolution in alkali.
Japanese Patent Publication (according to Chemical Abstracts 99/216843 u) No. 58 144 477 describes spray phosphating solutions for steel which contain 0.1 to 0.5 g/l of Zn, 15 to 30 g/l of phosphate and 0.01 to 0.2 g/l of nitrite. The phosphate layers thus produced are particularly suitable as the pretreatment for a subsequent cathodic electrocoating. Other prior publications include U.S. Ser. No. 373,475, now U.S. Pat. No. 4,419,199 which discloses activators for certain zinc phosphate solutions.
The disadvantages of the conventional processes with zinc concentrations of 0.1 to 0.6 g/l in the phosphating bath is that, when used for spraying on steel, they frequently lead to phosphate layers which are not level, and which are partly gray and partly have a greenish blue iridescent color. This unevenness may be evident in the subsequently applied electrocoating paint and may necessitate expensive after-treatment. Furthermore, the adhesion of the surface coating under load, for example as a result of bending or prolonged contact with water, does not always meet the requirements set.
It is the object of the invention to provide a process which does not have the disadvantages of the conventional processes and gives level, homogeneous phosphate layers with improved adhesion to surface coatings.